The present invention relates to processes for producing high-quality iron oxide products from a solution of ferrous salts selected from the group consisting of waste liquid containing ferrous salts and aqueous solution in which ferrous salts obtained from the waste liquid are dissolved.
More particularly, this invention relates to the above-mentioned process in which the waste liquid is an effluent from titanium dioxide manufacturing process or from sheet steel pickling process. Such waste liquid or effluent contains manganese compounds as impurities which are difficult to remove by a usual method.
At present, waste liquids containing ferrous salts and discharged from process for producing titanium dioxide or from process for pickling sheet steel pose environmental pollution problems with ever increasing severity.
Researches have long been made by various countries on the measures to abate the pollution problems as disclosed in the U.S. Pat. No. 2,529,874, Japanese Pat. Pubs. No. 14703/1963, No. 24351/1963, No. 24770/1968, etc.
However, the treatment of the above-mentioned waste liquids for purification requires enormous investment and expenditure, imposing a heavy economical burden on the industry concerned.
On the other hand, processes have heretofore been proposed for producing commercially useful iron oxide products from such waste liquids, but the conventional processes have various drawbacks given below.
For example chiefly employed for the production of iron oxide is a process in which ferrous sulfate having a relatively low manganese content as crystallized from waste liquid discharged from sheet steel pickling process or titanium dioxide manufacturing process or from waste sulfuric acid solution is thermally decomposed to synthesize iron oxide. (This process will be hereinafter referred to as "dry process") When ferrous sulfate is thermally decomposed at a temperature of up to 700.degree.C according to the dry process, the manganese sulfate contained in ferrous sulfate remains undecomposed (in water-soluble state), so that the manganese component can be removed without substantial difficulties by washing the resulting reaction mixture with water to obtain a ferric oxide product having a low manganese content.
Due to thermal decomposition, however, the dry process gives sulfur oxide emissions which cause serious air pollution. The immense investment and treatment cost required to control pollution therefore render the process almost infeasible. Furthermore, the dry process is limited to the synthesis of alpha-Fe.sub.2 O.sub.3 unless it is practiced under special production conditions, and the process is unable to yield black iron oxide of spinel structure (magnetite) and gamma-Fe.sub.2 O.sub.3.
For these reasons, another process has been introduced into use recently in which an alkali is added to a solution of ferrous salts such as ferrous sulfate and the resulting precipitate of ferrous hydroxide is oxidized with air to synthesize spinel-type iron oxide. (This process will be hereinafter referred to as "wet process") In addition to spinel-type iron oxide, the wet process produces a wide variety of iron oxides. For instance, spinel-type iron oxide gives gamma-Fe.sub.2 O.sub.3 when oxidized with air at a temperature of about 120.degree.C and yields alpha-Fe.sub.2 O.sub.3 when further roasted at a temperature of at least 450.degree.C. Moreover, the wet process has the outstanding advantage of being free of pollution experienced with the dry process owing to sulfur oxide emissions and permitting recovery of sulfur in the form of sulfate.
However, the known wet process also has the following difficulties.
Firstly, the spinel-type iron oxide prepared from a solution of manganese-containing ferrous salts using an equivalent or larger amount of alkali based on the ferrous salt solution according to the wet process invariably contains coprecipitated manganese compounds as impurities. Spinel-type iron oxide containing 0.2% or more of manganese oxides (calculated as MnO) is of poor quality for use as a black pigment, whilst ferric oxide prepared from such spinel-type iron oxide when used as a pigment fails to exhibit a satisfactory coloring ability, producing a turbid color which is low in lightness and saturation. Thus, the ferric oxide obtained is inferior in these important characteristics of pigment. Further when the above-mentioned spinel-type iron oxide is used as a material for ferrites impaired magnetic properties will result in the case of hard ferrite, while the production of manganese ferrite involves difficulties in determining the proportions of the ingredients, with the result that the starting material seriously deteriorates the commercial value of the products.
For the reasons given above, ferrous salts having a high manganese content are usable only for limited applications and are even dumped into the ocean if seldom, causing water pollution.
Secondly, typical of the wet processes heretofore adopted for producing spinel-type iron oxide is a process in which an alkali is reacted with an aqueous solution of ferrous salts and the resulting ferrous hydroxide is oxidized in a strongly alkaline aqueous solution with air or like oxidative gas introduced into the solution as disclosed in Japanese Pat. Pub. No. 668/1969. According to this process, the Fe.sup.2.sup.+ concentration of the aqueous reaction solution is limited to about 0.7 mol/l if highest and is usually at a lower level. Thus the Fe.sup.2.sup.+ concentration is much lower than that in the process of this invention to be described later which is 1.0 to 2.0 mols/l. The upper limit (0.7 mol/l) of Fe.sup.2.sup.+ concentration is critical to the conventional process because during the reaction between the ferrous hydroxide formed and aqueous solution of ferrous salts or as the oxidation reaction of ferrous hydroxide proceeds, the aqueous reaction solution tends to gel or exhibit increasing consistency, and this tendency inevitably becomes pronounced as the concentration of Fe.sup.2.sup.+ increases, making it difficult to permit the oxidative gas to act effectively. The low Fe.sup.2.sup.+ concentration required further entails difficulties in improving the operation efficiency. In addition, since the pH value of the aqueous solution to be reacted in the conventional process is at least 10 which is much higher than the pH value of 5 to 6 employed for the process of this invention to be set forth below, the aqueous solution obtained after iron oxide precipitate has been separated from the reaction mixture by filtration is strongly alkaline. However, it is not economical to recover sodium or like basic salts from the aqueous solution because they are dissolved therein in low concentrations, whereas if disposed of as it is, the solution will cause pollution.
Besides the generally practiced process described above, an attempt has been made to obtain spinel-type iron oxide Fe.sub.3 O.sub.4 by a special process in which a gaseous oxidative agent is continuously forced into an aqueous solution of ferrous salts having a pH of 4 to 5 and maintained at a high pressure of 4 to 10 kg/cm.sup.2 and at a high temperature of 140.degree. to 180.degree.C (Japanese Pat. No. 13456/1962), but this process requires a special equipment and is therefore unsuitable to produce spinel-type iron oxide inexpensively.
As will be apparent from above, the known process for producing iron oxide products from waste liquid from titanium dioxide manufacturing process, pickling liquor and like waste liquid containing ferrous salts give rise to incidental pollution problems, are very low in operation efficiency and require an enormous equipment investment, consequently increasing the cost of iron oxide products obtained. Moreover, due to the difficulties usually encountered in removing manganese compounds from the starting material in which they are inevitably contained as impurities these compounds remain in the end product, making it impossible to obtain high-quality products. These drawbacks render the conventional processes by no means feasible as processes for manufacturing iron oxide products which are also useful in purifying the waste liquids.